Aroon Up is 100 when today prints the highest high of the window and decays as time passes without a new high; Aroon Down does the same for lows. The indicator is about recency rather than magnitude, which makes it unusual.
Because it measures elapsed time, it can flag the end of a trend earlier than price-based smoothing: a market that stops making new highs registers immediately even if price has not fallen.
It is crude. A single marginal new high resets the reading completely, so it is sensitive to noise at the extremes, and it says nothing about how far price moved. Use it as a supplement to structure, not as a measure of trend quality.
Related: adx, lookback-period, donchian-channel, trend-following, market-structure